Accompanying the increasingly fine detail of circuit patterns to be projected in semiconductor fabrication, ever-higher resolution levels are being demanded from lithography exposure systems. As a result, light of shorter wavelengths is being used as the exposure light. In this specification, “light” is used not merely in the narrow sense of visible light, but rather has the broader meaning of “light” that includes electromagnetic radiation extending from so-called infrared rays to X rays, and having wavelengths shorter than 1 mm. In recent years, exposure systems have been proposed as next-generation lithography systems that use EUV (Extreme Ultraviolet) light with wavelengths of approximately 5 to 40 nm (hereinafter such systems are called “Extreme Ultraviolet Lithography exposure systems” or “EUVL exposure systems”).
In the extremely short wavelength range of EUV light, no materials are known that have sufficient transmissivity for use as refracting optical members. Consequently, a reflection-type projection-optical system, comprising only reflecting optical members, must be employed. In an EUVL exposure system, a reflection-type mask is used rather than a transmission-type mask, which requires that the illumination light be made incident obliquely to the mask. If the illumination light were made normally incident to a reflection-type mask, the optical path of the illumination light incident on the mask and the optical path of the illumination light reflected by the mask and propagating toward a projection-optical system would completely overlap. As a result, either the optical members of the illumination-optical system for illuminating the mask would block the optical path of the projection-optical system, or the optical members of the projection-optical system would block the optical path of the illumination-optical system.
In an EUVL exposure system, only a narrow arc-shaped effective exposure area (i.e., stationary exposure area) can be obtained using a reflection-type projection-optical system. Consequently, the mask and photosensitive substrate (wafer or analogous object) must be moved relative to the projection-optical system to scan the exposure light along the mask pattern over the photosensitive substrate (this is called “scanning exposure”). A field stop used for defining the stationary exposure area must be placed at a position that is substantially optically conjugate with the photosensitive substrate. In conventional EUVL exposure systems, a field stop is provided in the optical path of the illumination-optical system. By inserting an image-forming reflective optical system between the mask and the field stop, the field stop and the mask (and the photosensitive substrate) are placed in optically conjugate positions.
In general, the reflectivity per reflection surface is low in an EUVL exposure system. As a result, from the standpoint of avoiding loss of light, which reduces throughput, the number of reflections in the optical system between the light source and the photosensitive substrate (that is, in the illumination-optical system and in the projection-optical system) must be as few as possible. In conventional EUVL exposure systems, an image-forming reflective optical system is situated between the mask and the field stop as explained above. The resulting high number of reflections in the optical path of the illumination-optical system results in substantial loss of light, which prevents achievement of the necessary throughput.
To reduce the number of reflections in the optical path of the illumination-optical system and achieve the necessary throughput, a configuration may be adopted in which the field stop is positioned in proximity to the reflection-type mask. However, simply positioning the field stop in proximity to the reflection-type mask would cause a portion of the necessary radiation flux to be blocked by the field stop, with an adverse effect on the image-forming performance of the projection-optical system. This situation would raise the possibility that the mask pattern could not be transferred, accurately and without distortion, onto the photosensitive substrate.